A number of processes having as an object the preparation of olefin polysulfides for use as extreme-pressure additives for lubricants are disclosed in the prior art.
In particular, U.S. Pat. Nos. 3,471,404 and 3,697,499, disclose a process comprising the following main steps of:
(1) reacting sulfur monochloride with an excess of olefin having 2 to 5 carbon atoms in the molecule, particularly isobutene, at a temperature of 20.degree.-80.degree. C., so as to form an "adduct",
(2) reacting said adduct, obtained in the first step, with an alkali metal sulfide (preferably sodium sulfide) and elemental sulfur, used in a proportion of 1.8-2.2 moles of metal sulfide per gram-atom of sulfur, the proportion of alkali metal sulfide being of 0.8-1.2 mole per mole of adduct, and the reaction being performed in the presence of an alcohol or of a hydro-alcoholic solvent, at reflux, and
(3) reacting the obtained product, which contains 1-3% of chlorine, with an inorganic base in aqueous solution, at reflux, until the residual chlorine content of the product is lower than 0.5%.
These prior patents mention a sulfur content of the obtained products which might amount to 40-60% by weight. In fact it is mostly close to 46% by weight.
These products may be used as extreme-pressure additives for lubricating oils, transmission fluids or greases, the considered lubricating bases consisting of mineral oils and certain synthetic oils.
Furthermore, U.S. Pat. No. 4,204,969 discloses a rather similar process for preparing olefin polysulfides for use as extreme-pressure additives in lubricating oils. This process comprises the following main steps of:
(1) reacting sulfur monochloride, at about 30.degree.-100.degree. C., with a C.sub.3 -C.sub.6 aliphatic monoolefin (generally isobutene), preferably in the presence of a promoter consisting of a lower alcohol, so as to form an "adduct",
(2) reacting said adduct with sulfur and sodium sulfide (prepared for example from NaOH, NaHS and/or H.sub.2 S), in a proportion of 0.1-0.4 gram-atom of sulfur per mole of sodium sulfide, in a hydroalcoholic medium, at a temperature ranging from 50.degree. C. to reflux temperature, and recovering the obtained product without treatment by means of a base.
In the unique example of this patent, the indicated value of sulfur content is 49% by weight and that of viscosity at 37.8.degree. C. (100.degree. F.) is 8.6 mm.sup.2 /s (cSt).
When, according to the processes of the prior art, it is desired to increase the sulfur content of the additives by using an increased amount of elemental sulfur in proportion of the alkali sulfide or hydrogenosulfide, the obtained products are insufficiently soluble in synthetic lubricating oils (e.g of polyalphaolefin type) or even in mineral oils, to be used as extreme-pressure additives. Moreover, the kinematic viscosity of the obtained products is generally too high.
French patent applications Nos. 2 563 231 and 2 571 380, considered as a whole, or the European patent application No. 159 936, disclose a process for preparing olefin polysulfides, substantially defined by the following steps of:
(1) reacting at least one compound selected from sulfur monochloride and dichloride with at least one monoolefin having from 2 to 5 carbon atoms (generally isobutene) in a proportion of 1.5-2.5 moles of monoolefin per mole of sulfur monochloride and/or dichloride, thus forming an addition product called "adduct",
(2) contacting said adduct with at least one hydrocarbyl halide selected from chlorides, bromides and iodides of C.sub.1 -C.sub.12 alkyls, C.sub.5 -C.sub.12 cycloalkyls or substituted cycloalkyls and C.sub.6 -C.sub.12 arylalkyls or substituted arylalkyls, the amount of said hydrocarbyl halide corresponding to 1-70%, in halogen gram-atoms, in proportion to the number of halogen gram atoms of the aggregate amount of said adduct and said hydrocarbyl halide (i.e. 0.015-1.9 halogen gram-atom per 100 g of adduct), with at least one sulfur compound selected from alkali metal sulfides, hydrogenosulfides and polysulfides of ammonium or alkaline-earth metals, used in a proportion of about 0.4-0.8 mole per halogen gram-atom containing in the aggregate amount of adduct and hydrocarbyl halide, and a proportion of elemental sulfur from 0 to 7 gram-atoms per mole of said sulfur compound, with a medium consisting of water or of a mixture of wter with aliphatic monoalcohol,
(3) heating the resultant mixture and, after separation into two phases, recovering the olefin polysulfide in the organic phase, and
(4) optionally treating the product obtained in step (3) with a basic compound such as an inorganic base.
In these prior patent applications it is stated that the prepared products are olefin polysulfides, whose sulfur content may reach about 45-65 % by weight. Their kinematic viscosity at 100.degree. C. varies in relation with their sulfur content. It may be about 4-20 mm.sup.2 /s. Their halogen (mainly chlorine) content is generally lower than about 1% by weight and mostly than 0.6% by weight.
Their solubility in lubricating oils, particularly in hydrogenated polyalphaolefins (PAO) depends on the operating conditions used for their preparation.
U.S. Pat. No. 4,563,302 discloses a process which may also produce olefin polysulfides of increased solubility in polyalphaolefins (up to 8% per weight). This process comprises the following main steps of:
(1) reacting a sulfur halide such as sulfur monochloride S.sub.2 Cl.sub.2, sulfur dichloride SCl.sub.2 or a mixture of the two reactants, with an aliphatic olefin containing 3-6 carbon atoms, so as to obtain an addition product called an "adduct",
(2) reacting said adduct with sulfur, sodium sulfide Na.sub.2 S, an alkyl mercaptan having 1-12 carbon atoms and optionally sodium hydrosulfide NaSH, in a hydroalcoholic medium within a temperature range from 50.degree. C. to reflux temperature, so as to form said olefin sulfide, and
(3) separating said olefin sulfide from the hydroalcoholic medium.
U.S. Pat. No. 3,873,454 discloses an extreme-pressure additive for lubricants obtained by reacting isobutylene with a sulfur halide so as to form an adduct which is then reacted with an alkali metal mercuptide in an inert medium, thus forming a compound of the formula: ##STR1##
The alkali metal mercaptide may be sodium, potassium, lithium or calcium mercaptide. The reaction medium is generally a lower alcohol (C.sub.1 -C.sub.4). The desired compound (having a sulfur content of about 53%) is obtained with a yield of about 45 % by weight, the remaining 55% consisting of a mixture of unsaturated sulfides and polysulfides.
In the unique example of preparation of the compound it is stated that a dispersion of solids is obtained by stirring a mixture of sodium mercaptide in ethanol, before adding the adduct, separately prepared.
The desired product is finally obtained as a solid, which is separated and purified.
However, such a compound is not soluble to a large extent in mineral oils and in synthetic oils of the polyalphaolefin type.